Use the button's "HyperlinkAddress" property...
Here's some sample code that launches and verifies the syntax of a website entered in a corresponding text box:
'***
Private Sub btnLaunchWebsite_Click()
If Left(edURL1.Value, 11) = "http://www." Then
btnLaunchWebsite.HyperlinkAddress = edURL1.Value
ElseIf Left(edURL1.Value, 4) = "www." Then
btnLaunchWebsite.HyperlinkAddress = "http://" & edURL1.Value
Else
MsgBox "Please enter the website in one of the following formats: " & _
vbCrLf & vbCrLf & _
"www.blah.bla" & vbCrLf & "http://www.blah.bla" & vbCrLf & "http://www.blah.bla/", vbExclamation, "Unable to Launch Website..."
End If
End Sub